1. Field of the Invention
The present invention relates to a method for removing the small amounts of .sup.14 CO.sub.2, .sup.14 CO and corresponding alkanes produced in nuclear power stations from the exhaust gases of the purification plants.
2. Description of the Prior Art
In most nuclear power stations, water is used as the coolant. It is unavoidable in pressurized-water reactors and in boiling-water reactors that radioactive impurities, which may also be of a gaseous nature, get into the cooling water loop or are formed there. It is therefore customary to always branch off part of the circulating water from the main coolant loop and to conduct it through a purification plant, to remove the radioactive impurities there, to degas the water and then return it to the main coolant loop. This known technique is schematically shown for a pressurized-water reactor in FIG. 1 and for a boiling-water reactor in FIG. 2. By the extremely high radiation density in the reactor core, a very small amount of water is furthermore dissociated radiolytically into hydrogen and oxygen. In the degassification station of the purification plant, these gases are likewise liberated and changed catalytically into water again in a recombination arrangement. In this manner, the development of an ignitable hydrogen-oxygen mixture is prevented from the start. The remaining exhaust gases are customarily transported over a bed of activated carbon, where they are adsorbed, lose most of their radioactivity during the storage time and are discharged after delay into the outside air via the exhaust air stack.
The traces of radioactive carbon .sup.14 C which is contained in the exhaust gases and has a half-life of more than 5000 years, are discharged to the outside via the stack practically unchanged. The formation of this radioactive carbon isotope is derived from the (n,.alpha.) reaction with the oxygen isotope of the water, .sup.17 0, and also from the (n,p) reaction with possible nitrogen contaminations. This radioactive carbon is present substantially as monoxide, dioxide and as alkane.
Although only small amounts of this radioactive carbon are formed, it might become necessary with the expected increased energy production via nuclear power plants, because of the biochemical importance of this carbon isotope, to no longer discharge the latter into the free atmosphere but to collect it and to add it to the radioactive wastes.